The use of structurally hardened core alloys for heat exchangers (particularly in the 6xxx series: Al—Mg—Si) was very frequent as long the vacuum brazing type process was used. This practice was terminated when the brazing technology changed and was replaced by brazing under a controlled atmosphere with Nocolok® non-corrosive flux, related to the high acquisition and maintenance costs of vacuum furnaces. The Nocolok® process imposes strict constraints on the use of magnesium alloys, since magnesium reacts with the flux used to dissolve the oxide layer, and makes it inoperative. The limit for the content is of the order of 0.3%. A very large amount of the flux would be necessary for higher contents, which would make the operation extremely expensive.
Moreover, the resistance to corrosion of a large number of strips for exchangers is based on the formation of an anodic layer at the core/cladding interface which imposes a very low content of silicon in the core. For example, this is the case of alloys described in patent EP 0326337 (Alcan).
Therefore 6xxx alloys were very largely replaced by 3xxx alloys with low magnesium and low silicon contents and the structural hardening effect was lost.
Type 3xxx core alloys with structural hardening were recently proposed, for example in patent EP 0718072 (Hoogovens Aluminum Walzprodukte) and in patent application EP 1254965 (SAPA Heat Transfer). In both cases, no modification has been made to the strips to improve their brazability in a standard Nocolok® furnace. Consequently, either the magnesium content must be limited to a relatively low value (for example less than 0.35% as in the case of application EP 1254965), but the structural hardening effect is then reduced relatively small, or the quantity of flux deposited has to be increased or an alternative flux has to be used such as cesium flux described in U.S. Pat. No. 5,771,962 (Ford). In both cases, this significantly increases the cost of the operation.
The invention is intended to propose a material that has structural hardening properties, and also good brazability in existing Nocolok® lines.